Unfortunately, the notifications WebExtension API I'm forced to use now is kinda not-great (also note, apparently the icon gets stretched on some platforms), and Mozilla hasn't even fully implemented it. I used to have a "Don't show this again" checkbox, but there's currently no way to add a custom checkbox/button here. I may have to write my own notification popup from scratch to add this feature back, as unfortunately is a recurring pattern when dealing with this crap API. This is not a simple thing to do, so it's lower down on the todo list, at the moment. The notifications should still only show once per session, though.

The actual setting to remember the old checkbox still exists, so technically you could manually edit the settings file (without Firefox open) and hack it in. It's just JSON, so you could add the following to the end of the file, but before the last curly brace ("}"):

(note the comma; spaces around commas/colons don't matter)
The FAQ has instructions on hunting down the settings location here.

Mozilla has said that they intend to add some sort of GUI to allow people to edit WebExtension settings, similar to about:config... however, they haven't (and I don't know if they actually will, unfortunately).